Frequency Modulation (FM) detection, using a so called quadrature detector, is well known. This kind of FM detector typically is a four quadrant multiplier with one input terminal driven by an amplitude limited FM modulated information signal (reference signal), and another input terminal driven with a version of the reference signal which is quadrature (90.degree.) phase shifted relative to the reference signal using a tuned circuit including an RLC network. With a single tuned circuit, the resultant transfer characteristic of the detector, while closely approximating a straight line near the center (carrier) frequency of the modulated signal, departs from a straight line when the frequency of the FM signal deviates appreciably. Because of the wide deviation of a conventional FM broadcast signal (.+-.75 KHz), the departure from a straight line introduces harmonic distortion, particularly for louder audio signals.
The slope of the phase versus frequency curve of the tuned circuit is determined by its "Q". Distortion can be reduced by lowering the "Q" but this reduces the output signal level so that a reduced signal-to-noise (S/N) ratio results. This means that the quadrature FM detector requires a linearity versus sensitivity trade-off between total harmonic distortion (THD) and S/N.
Tolerances in the values of the inductance and capacitance of the tuned circuit can result in a phase versus frequency curve that is not centered at the FM carrier frequency. Although the demodulator will still function under such non-ideal conditions, less of the linear portion of the phase curve will be available and the THD will be degraded. This degradation of THD can be compensated for by lowering the "Q" of the tuned circuit. However, as stated above, the result would be to degrade S/N performance. In order to optimize THD while still maintaining a reasonably high "Q" for good S/N, an alignment of the tuned circuit is desirable so that it is accurately tuned to the FM carrier frequency. Current manufacturing processes and apparatus control systems make it desirable that such an alignment be performed electronically since the operation of currently produced apparatus, e.g., television receivers and the like, are typically bus controlled under the control of a programmed microprocessor and a PROM memory.